xenapi: remove pv detection

It was assumed that pygrub is there in domU, so that the RAW and unknown
disks were attached to domU and investigated with pygrub to find out if
PV could be used. This patch removes this functionality. Also some
cleanup is made around the ImageType mess.

From now RAW and unknown disks will not be investigated for PV-ness and
will boot as HVM.

Fixes bug 1196570

Change-Id: If84b93cee36938257b7ad0fa44b3b22878520506
Merge "xenapi: remove pv detection"